Patent GB-906546-A (GB)
Title: Improvements in or relating to correction of uncorrected colour extracts
IPC: H04N1/60
Filed: 1958-11-04
Abstract: 906,546. Local copying in colour. HELL, R. Nov. 4, 1958 [Nov. 4, 1957], No. 35366/58. Class 40 (3). [Also in Group XXXVI] The invention relates to electrical computing apparatus for the derivation of three colour components B, R, G for use in a colour reproduction process from three colour components X, Y, Z of an original in accordance with the relations: where the form of the functions b, r and g is determined by the character of the reproduction colours, printing paper and printing process. In accordance with the invention, the processes of computation are simplified by resolving the three three-variable functions b, r and g into nine two-variable functions b 1 , b 2 , b 3 , r 1 , r 2  . . . g 3  based on the equations: The functions b 1 , b 2 , b 3 , &c., which are determined empirically for particular printing colours, papers and processes by printing sample colours in all possible densities, have general characteristics which are independent of the actual colours, papers and processes. Typical curves for the functions for an intaglio printing process are illustrated in Figs. 1-9 (not shown) In this particular process three of the functions reduce to one-variable functions. The computer, Fig. 10, consists of three channels A, B and C which receive respectively voltages representative of X, Y and Z and derive voltages representative of B, R and G. The voltages X, Y, Z may be derived by photo-electric point-bypoint scanning of the coloured original or component colour photographic separations of the original. The derived voltages B, R, G may be employed to control either recording lamps for photographic reproduction or engraving tools for printing plate production. Each channel of the computer comprises: (a) a first stage (7, 8, 9) in which the channel input voltage has subtracted from it a voltage representative of the function (b 1 , r 1 , g 1 ) of the other two channel input voltages, the voltage being derived by a first function generator (16, 17, 18), (b) a linear amplifier (10, 11, 12) whose gain is controlled by a second function generator (19, 20, 21) so as to cause the signal from the first stage to be multiplied by the function (b 2 , r 2 , g 2 ) of the other two channel input voltages, and (c) a function generator (13, 14, 15) which is controlled jointly by the output of the amplifier and the input voltage of one other channel so as to produce finally the desired colour component (B, R, G) in accordance with the third function (b 3 , r 3 , g 3 ). The function generators may comprise a cathode-ray tube whose beam is deflected in the horizontal and vertical directions respectively by the two voltages and whose spot is imaged optically on to a photoelectric cell through a mask having a density distribution in accordance with the function. Reference is also made to the use of cathode-ray tubes with specially constructed targets and non-linear circuits. The outputs from the computer may be connected into four-colour (i.e. three colours and black) or six-colour form before use. U.S.A. Specification 2,434,561 is referred to.